Maintaining Stormwater Management Ponds

Fairfax County faces many challenges in controlling the quantity and
quality of stormwater runoff. The way stormwater is managed has a
direct affect on the health and stability of streams, the primary
receptacles of stormwater runoff. Land development brings with it more
impervious (nonporous) surface which increases the amount of runoff.
Despite the County’s best efforts to protect the natural environment from
the effects of development, our streams still suffer from excess erosion,
sedimentation, and pollution. One way of controlling increased runoff
is to build stormwater management facilities.

Stormwater management (SWM) in Fairfax County typically involves
ponds. A pond intercepts the runoff before it reaches a stream. The
term “pond” may confuse those of us not in the business of stormwater
management because the term conjures up an image of a permanent pool of
water. However, a stormwater management pond can be either wet or dry. A
wet pond is exactly that. It is a basin or depression that
retains, or holds, water in a permanent pool. While the term “dry
pond” sounds like an oxymoron, it refers to a basin or depression
that detains, or slows, the flow of water for short periods of time and
is dry between storm events. Wet ponds are often aesthetically pleasing
to the eye and may provide recreational opportunities. Dry ponds may look
less attractive or go completely unnoticed in the landscape. Whether
wet or dry, SWM ponds serve an important purpose. They control the volume
of runoff by releasing it over time. Every pond has a pipe outlet.
The outlet is generally sized to release water over a 2-3 hour period in
a heavy storm and less time or none at all in light precipitation. If an
increase in runoff is not controlled, it may cause downstream flooding
and stream bed and bank erosion.

Some SWM ponds control not only the quantity of runoff but also the
quality of runoff. In such cases, the SWM ponds are called BMP ponds.
BMPs, or best management practices, are techniques to manage runoff in
ways that reduce water pollution. In a BMP pond, a flow regulator is
attached to the end of the pipe to reduce the size of the outlet. A
smaller outlet forces the pond to hold the water for a longer period,
allowing more time for the sediment and attached nutrients to settle out.
Whereas a conventional SWM pond will release stormwater over 2-3 hours, a
stormwater management BMP pond may release the water over 2-3 day days.

What must an SWM pond accomplish? State regulations require that
the volume of post-development (after construction) runoff must be equal
to or less than the volume of pre-development (before construction)
runoff as measured against a unit of time. In other words, a property
downstream of a new development must not see more runoff per second than
it did before the development took place. A properly functioning SWM pond
spreads out the flow of runoff over time to control flooding and erosion.

In Fairfax County there are 863 SWM ponds maintained by the Department
of Public Works and Environmental Services (DPWES), of which 317 are BMP
ponds. The efficiency of the stormwater management system is dependent on
how well the ponds are maintained.

The County expends a great deal of human and financial resources to
maintain the ponds. But even with a high quality maintenance program,
some failures still occur. Replacing a failed dam can cost in excess of
$300,000.

SWM ponds fail for a number of reasons. The condition of the dam,
which holds the water in the pond, is one cause of pond failure. As
water from the pond seeps into the earthen dam, the dam, if not
constructed properly, could weaken to the point of washing out. The soil
used to construct the dam must be properly selected and compacted so that
it will remain in place while the pond functions. Special care must be
taken to ensure that the pipe passing through the dam does not leak or
allow seeping water to erode the soil around the pipe. Water will always
take the path of least resistance, meaning that it will find its own
pathways through a poorly constructed dam.

In 1995 the County adopted improved dam standards that address the
seepage and leak problems. Prior to 1995, the pieces of pipe going
through the dam were fitted together without a seal. Today, all pipe
joints must have a rubber gasket to prevent leaks. In addition, pipes
must sit on a concrete cradle to eliminate any gaps below or on the sides
of the pipe. Around the pipe and cradle there must be gravel which is
wrapped in a filter material. This forms a drainage blanket. Any water
that does seep through the dam will pass through the drainage blanket. A
perforated pipe from the drainage blanket carries the filtered water to
the stream.

Another problem affecting the function of ponds is the condition of
the trash rack. In both dry and wet ponds, there is a screening
device attached to the outlet which prevents trash and other debris from
leaving the pond and entering the stream. If the trash rack gets so
clogged with litter, leaves, and grass clippings that the water cannot
freely pass through it, then the outlet will not function adequately. As
a result, the water will back up behind the dam. Since the dams on dry
ponds were not designed to hold back a pool of water for an extended
period, the dam may wash out.

Not all SWM ponds are maintained by the County. There are some that are
privately maintained by homeowner associations or corporate entities. A
different type, but no less important, of a challenge for DPWES stems
from the role that property owners play in the operation and maintenance
of these SWM ponds.

For more information about stormwater management ponds, contact the
DPWES Maintenance and Stormwater Management Division at 703-324-5500, TTY
711.